How many calories should I eat per day?

The number everyone quotes is about 8,700 kilojoules a day in Australia — roughly 2,000 Calories — and it's a fine starting point. But it hides two things: that figure is a population average, not a personal target, and the calorie counts printed on the food itself are 120-year-old estimates that don't match what your body absorbs.

People want a single tidy number. The honest answer has two parts — what the reference figure actually is, and why even a precise-looking calorie count is softer than it looks. Start with the widget: pick a food and watch how much of its label energy never reaches you.

So what is the number?

In Australia the working figure is 8,700 kilojoules a day. It is the reference the law puts on fast-food menu boards — every large chain has to display the line "The average adult daily energy intake is 8,700 kJ" — and the government's healthdirect service uses the same number as the rough amount an average adult needs to hold a steady weight. The United States and Canada build their label percentages around a 2,000-Calorie diet instead, which is about 8,360 kJ.

The key word is average. That figure is a population midpoint; your own number moves with age, body size, sex, how active you are and life stage. If you want a figure genuinely tuned to you, that's a conversation for a doctor or an accredited dietitian — not something a web page can hand you.

Where "2,000 calories" actually came from

The "2,000 calories" on a nutrition label isn't advice — it's a rounded reference value, chosen so the percentage figures on the panel have something to divide by. 8,700 kJ works out to roughly 2,080 Calories, and 2,000 is simply the tidy number to remember it by. Worth knowing too: the "Calorie" with a capital C on a food label is really a kilocalorie — a thousand of the small calories from a physics textbook.

The label calorie is an estimate, not a measurement

Here's the part most people never hear. The calorie count on a label isn't measured for that specific food — it's calculated. The protein, fat and carbohydrate are each multiplied by a fixed energy value: about 4, 9 and 4 kilocalories per gram. Those are the Atwater factors, worked out by the chemist Wilbur Atwater around 1900. They are averages, and they quietly assume your body extracts all of that energy.

It often doesn't. USDA researchers Janet Novotny and David Baer measured the real figure by feeding whole foods to people under controlled conditions. Whole raw almonds came in around 129 kcal a serving against an Atwater estimate of 168–170 — close to a third of the label energy never absorbed. Walnuts measured about 21% below their label. And as the widget above shows, the gap shrinks the more the food is broken down: a roasted almond gives up more than a raw one, a chopped almond more again, and almond butter — every cell wall ground open — releases almost the full label amount.

Digesting food has its own energy cost

Even the energy you do absorb isn't all yours to keep. Digestion costs energy — breaking food down, absorbing it and storing it burns calories of its own. This thermic effect of food is usually put at around 10% of what you eat, and it isn't equal across foods: protein is expensive to process, carbohydrate and fat much less so. So 100 label-calories of chicken breast leave you with measurably less net energy than 100 calories of soft drink. (A widely repeated "eat 10% over your basal rate" claim seems to trace back to this digestion cost rather than to any single study — if you have an original source for it, it's worth checking.)

What about glucose spikes?

Recently the question picked up a new layer: blood-sugar spikes. The popular version, associated with Jessie Inchauspé — the "Glucose Goddess" — argues that flattening the glucose rise after meals matters for everyone, not just people with diabetes. Some of it is sound. Pairing carbohydrates with protein, fat or fibre genuinely does slow the rise, and a savoury breakfast over a sugary one is reasonable advice.

But dietitians point out that this is ordinary, long-standing nutrition guidance, not a discovery. The contested parts are the stricter claims — rigid food-ordering rules and vinegar "hacks" rest on thin evidence, and in people without diabetes blood glucose is already tightly regulated and naturally rises and falls through the day. Inchauspé trained in biochemistry rather than dietetics, and nutrition professionals, including McGill University's Office for Science and Society, have pushed back on how far the claims get stretched. Useful ideas, oversold.

Treat both numbers as estimates with error bars. The daily figure is a population average wearing your name, and the label calorie is a 120-year-old calculation that your fibre, your cooking and your own gut quietly edit. Calories are a useful map — just not the territory.